CVE-2023-38709: Apache HTTP Server XSS Vulnerability

CVE-2023-38709 Overview

CVE-2023-38709 is an HTTP Response Splitting vulnerability affecting Apache HTTP Server through version 2.4.58. The vulnerability stems from faulty input validation in the core of Apache that allows malicious or exploitable backend/content generators to split HTTP responses. This flaw can be exploited by attackers to inject arbitrary HTTP headers and content into server responses, potentially leading to cache poisoning, cross-site scripting attacks, or session hijacking.

Critical Impact

Malicious actors can exploit this vulnerability to manipulate HTTP responses, enabling cache poisoning attacks, cross-site scripting (XSS), and potential session hijacking across affected Apache HTTP Server deployments.

Affected Products

• Apache HTTP Server through version 2.4.58
• Debian Linux 10.0
• Fedora 38, 39, and 40
• NetApp ONTAP 9
• NetApp ONTAP Tools 10 for VMware vSphere
• Broadcom Fabric Operating System
• Apple macOS

Discovery Timeline

• April 4, 2024 - CVE-2023-38709 published to NVD
• November 4, 2025 - Last updated in NVD database

Technical Details for CVE-2023-38709

Vulnerability Analysis

The vulnerability exists in the core input validation mechanisms of Apache HTTP Server. When processing responses from backend servers or content generators, Apache fails to properly sanitize certain control characters and line terminators that can be used to inject additional HTTP headers or split the response into multiple parts.

HTTP Response Splitting occurs when an attacker can inject carriage return (CR) and line feed (LF) characters—represented as \r\n—into HTTP response headers. These characters are interpreted by HTTP parsers as header delimiters, allowing the injection of arbitrary headers or even an entirely new HTTP response body. In the context of CVE-2023-38709, this faulty validation allows backend applications or content generators that have been compromised or are otherwise malicious to craft responses that, when passed through Apache, result in split or manipulated responses being sent to clients.

Root Cause

The root cause is classified under CWE-1284 (Improper Validation of Specified Quantity in Input). The Apache HTTP Server core does not adequately validate or sanitize input from backend content generators before incorporating it into HTTP responses. Specifically, the validation routines fail to properly detect and reject or encode control characters that have special meaning in the HTTP protocol, particularly the CR/LF sequences used to delimit headers.

Attack Vector

This vulnerability is exploitable over the network without requiring authentication or user interaction. An attacker who can influence the output of a backend content generator—either by compromising the backend application, exploiting another vulnerability in the backend, or controlling a proxied upstream server—can inject malicious content into the HTTP response stream.

The attack flow typically involves:

1. Attacker gains influence over a backend application or content generator proxied by Apache
2. Malicious response content containing CR/LF sequences is crafted
3. Apache HTTP Server passes the malicious content without proper sanitization
4. The client receives a manipulated or split HTTP response
5. Cache poisoning, XSS, or session manipulation attacks can then be executed

For detailed technical information on the exploitation mechanics, refer to the Apache HTTPD Security Advisory and the Openwall OSS-Security Mailing List discussion.

Detection Methods for CVE-2023-38709

Indicators of Compromise

• Unusual HTTP response headers appearing in server logs or client-side monitoring
• Unexpected Set-Cookie headers or modified Location headers in responses
• Evidence of cache poisoning where cached responses contain unexpected content
• Backend application logs showing injection attempts with encoded CR/LF characters (%0d%0a or \r\n)

Detection Strategies

• Monitor HTTP access logs for responses with abnormal header counts or unexpected header values
• Implement Web Application Firewall (WAF) rules to detect CR/LF injection patterns in upstream responses
• Use network intrusion detection systems (IDS) to identify HTTP response splitting signatures
• Deploy application-level monitoring to track anomalies in backend response handling

Monitoring Recommendations

• Enable detailed logging of HTTP response headers in Apache configuration
• Implement real-time alerting for cache-related anomalies and unexpected response modifications
• Monitor backend application health and integrity to detect potential compromise
• Configure security information and event management (SIEM) rules to correlate HTTP response anomalies across the infrastructure

How to Mitigate CVE-2023-38709

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.59 or later immediately
• Review and audit all backend content generators and proxied applications for security issues
• Implement input validation at the application layer for all content passed to Apache
• Enable additional logging to monitor for exploitation attempts during the patch window

Patch Information

Apache has released version 2.4.59 which addresses this vulnerability. Organizations should apply the update as soon as possible. Detailed patch information and download links are available from the Apache HTTPD Security Vulnerabilities page.

For distribution-specific updates:

• Debian users should reference the Debian LTS Security Announcement
• Fedora users can find package updates in the Fedora Package Announcements
• NetApp customers should consult NetApp Security Advisory NTAP-20240415-0013
• Apple macOS users should review Apple Support Article HT214119

Workarounds

• Implement strict input validation at the application layer to reject responses containing CR/LF sequences
• Deploy a Web Application Firewall (WAF) with rules to detect and block HTTP response splitting attempts
• If using Apache as a reverse proxy, consider adding an intermediate sanitization layer for backend responses
• Restrict backend content generators to trusted, hardened applications only

# Verify Apache version and check for vulnerability
httpd -v
# or
apache2 -v

# For systems using package managers, check installed version:
# Debian/Ubuntu
dpkg -l | grep apache2

# RHEL/CentOS/Fedora
rpm -q httpd

# Update to patched version:
# Debian/Ubuntu
sudo apt update && sudo apt upgrade apache2

# RHEL/CentOS/Fedora
sudo dnf update httpd